The term “rapid prototyping” (RP) covers generative manufacturing processes in which 3-dimensional models or components are prepared from computer-aided design data (CAD data) (A. Gebhardt, Vision of Rapid Prototyping, Ber. DGK 83 (2006) 7-12). These are processes, such as stereolithography (SL), selective laser sintering (SLS), 3D printing, fused deposition modelling (FDM), ink-jet printing (IJP), 3D plotting, multi-jet modelling (MJM), solid freeform fabrication (SFF), laminated object manufacturing (LOM), laser powder forming (LPF) and direct ceramic jet printing (DCJP), with which models, components or shaped parts can be prepared cheaply even on a small scale (A. Gebhardt, Generative Fertigungsverfahren, 3rd edition, Carl Hanser Verlag, Munich 2007, 77 et seq.). Stereolithography involves RP processes (A. Beil, Fertigung von Mikro-Bauteilen mittels Stereolithographie, Düsseldorf 2002, VDI-Verlag 3 et seq.) in which a shaped part is constructed in layers from a liquid and curable monomer resin on the basis of CAD data.
Stereolithographic processes for the preparation of dental shaped articles, such as inlays, crowns or bridges, are highly advantageous particularly with ceramic materials, because the impression-taking and casting processes and the grinding and milling operations, respectively, which involve considerable manual outlay in the dental laboratory, can thus be greatly simplified and at the same time the large material loss which occurs with non-generative processes can be avoided. As a complete digital process chain is in place today, the standard process steps for the preparation of e.g. multi-unit bridge frameworks (alignment in the articulator, wax modulation, embedding and casting) can be replaced by the digitalization of the model, the virtual design of the dental shaped article and its generative stereolithographic manufacture.
In the stereolithographic preparation of ceramic shaped parts a ceramic green compact is firstly prepared by radiation curing layer-by-layer of a free-flowing ceramic slip which is then sintered after debinding to form a dense ceramic shaped article. The green compact is also referred to as a green body. The term debinding is used to describe the elimination of the binder. In this step, the binder employed is usually removed by heating the green compact to a temperature of about 90° C. to 600° C. It is essential that the formation of cracks and deformations are very largely avoided. The green body becomes the so-called white body as a result of the debinding.
In debinding, purely thermal as well as thermochemical processes take place. Mixtures of water, solvents, polymers, waxes or oils are usually employed as binders in the pressing of ceramic powders. Polypropylene, polyethylene, polyvinyl acetate, polyvinyl alcohol, methylcellulose, polyvinylpyrrolidone, polystyrene or polyethyl methacrylate are most often used as polymers (cf. R. Moreno, Amer. Cer. Soc. Bull. 71 (1992) 1647-1657). These are linear polymers which are broken down more or less easily at increased temperature into volatile components through depolymerization or chain-splitting.
The sintering of the white body takes place in the sintering furnace during high-temperature firing. This results in a compacting and solidifying of the finely-dispersed ceramic powder by exposure to a temperature below the melting temperature of the main components, whereby the porous body becomes smaller and its strength increases.
EP 1 021 997 A2 and US 2005056350 (A1), which is hereby incorporated by reference, describe the use of the laser-sintering process for the preparation of dental restorations. Here, metal powders are sintered in layers using a laser. US
DE 101 14 290 A1 and U.S. Pat. No. 6,939,489, which is hereby incorporated by reference, relate to the preparation of dental shaped parts by 3D plotting using materials that are meltable, condensable, curable thermally or with UV or visible light, filled or unfilled. For the preparation of green bodies inorganic pastes are proposed which are composed of glass, glass ceramic or ceramic powder, which is converted into a shapable paste with solvent, binder and plasticizer. The powders used are not surface-modified.
WO 97/29901, which is hereby incorporated by reference, describes a process and an apparatus for the preparation of 3-dimensional components from a liquid, curable medium. The component is constructed in layers by scanning each individual layer with a laser and curing it. The next layer of the curable material is then deposited by means of a coating device and then likewise cured.
A stereolithographic process for the preparation of dental implants is known from WO 95/28688 and U.S. Pat. No. 5,768,134, which is hereby incorporated by reference.
U.S. Pat. No. 5,496,682, which is hereby incorporated by reference, discloses light-curable compositions for the preparation of three-dimensional bodies by stereolithography, which contain 40 to 70 vol.-% ceramic or metal particles, 10 to 35 wt.-% monomer, 1 to 10 wt.-% photoinitiator, 1 to 10 wt.-% dispersant and preferably also solvent, plasticizer and coupling agent.
U.S. Pat. No. 6,117,612, which is hereby incorporated by reference, describes resins for the stereolithographic preparation of sintered ceramic or metal parts. The resins have a viscosity of less than 3000 mPa·s. For their preparation, monomers with a low viscosity are used, preferably in aqueous solution. A high solids content and low viscosity are said to be achieved through the use of dispersants.
DE 10 2005 058 116 A1, which is hereby incorporated by reference, discloses suspensions for the stereolithographic preparation of ceramic implants in the manner described in U.S. Pat. No. 6,117,612, which is hereby incorporated by reference, which do not contain diluents such as water or organic solvents, as the latter are said to increase the viscosity through local evaporation when energy is introduced. The viscosity of the suspension is adjusted to less than 20 Pa·s by varying the concentration of a dispersant. Alkyl ammonium salts of copolymers with acidic groups are used as dispersants, which can also be coated onto the particles of the ceramic powder.
Methods and compositions for the stereolithographic preparation of ceramic components are described in US 2005/0090575 A1, which is hereby incorporated by reference. It is stated that shaped parts prepared with the liquid materials known from U.S. Pat. No. 5,496,682, which is hereby incorporated by reference, are soft and therefore require an additional curing step in order to avoid deformations during firing, while shaped articles obtained from paste-like materials develop internal stresses during debinding which lead to cracks during sintering. To avoid these problems, plasticizers are used and the quantity of ceramic powder is chosen such that the viscosity of the compositions is at least 10,000 Pa·s.
Compositions curable with visible light and their use for the preparation of dental restorations from plastic materials by RP processes are described in DE 199 38 463 A1 and DE 199 50 284 A1, which are hereby incorporated by reference.
For the preparation of high-strength ceramics, the process chain must be carried out in such a way that cracks, separated layers, pores and other defects or deformations do not occur during the preparation of the green body, brown body and white body. In this regard, the composition and the properties of the radiation-curable slip are of decisive importance. Thus as high as possible a proportion by volume of the ceramic particles in the slip is required in particular for a high density and final strength as well as good accuracy of fit of the ceramic shaped part. Furthermore, a properly adjusted rheology of the slip is a basic requirement for a rapid and problem-free stereolithographic construction of a green body with few defects, wherein however the viscosity and the flow behaviour depend among other things on the size and the content of the ceramic particles in the slip as well as on the type and quantity of added rheology additives.
The stability of the stereolithographically produced green bodies can be improved by using cross-linking monomers. The curing time which is required to obtain a stable solid can thereby be significantly shortened. However, at the same time, the polymer network that forms also displays a much higher thermal stability compared with linear polymers, which adversely affects the debinding process. Moreover, the polymerization shrinkage increases with the functionality of the monomers which can lead to considerable shrinkage stresses. As a result, green bodies, brown bodies and sintered white bodies which have porosities and defects are often obtained even during optimized processing methods. A particular problem with the stereolithographic preparation of shaped articles is that the individual layers do not adhere to each other sufficiently, which can lead to delamination.
The known slips fail to satisfy the above-named requirements. The object of the invention is therefore to provide light-curing ceramic slips for the stereolithographic preparation of ceramic shaped parts, which display a good stereolithographic processability, result in sufficiently strong and dimensionally stable green bodies and, after debinding and sintering, lead to defect-free, high-strength ceramics. In particular, delamination is to be prevented.